JP-B-39-8547 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses an improved adhesive for electric appliances, which contains an aqueous silica sol (such as silica powder) having a particle size of from 5 to 150 microns and metal oxide such as boron oxide, lead oxide, antimony troxide or the like.
JP-A-48-93636 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a composition comprising an acidic silica sol having a particle size of from 7 to 150 millimicrons, an aqueous solution for crystalloid silicate which is free from alkali metal ion and an inorganic refractory powder.
JP-B-56-38544 discloses an inorganic composition comprising an aqueous silica sol and an inorganic fiber or refractory powder having a particle size of from 0.1 micron to 20 microns along with pigment, aggregate and other aids.
Hardened products of adhesive compositions containing a silica sol and a refractory powder, but not containing any vitrifying components, lack sufficient adhesive strength though displaying a high heat-resistance.
An adhesive comprising from 20 to 70 parts of an aqueous silica sol having a concentration of from 2 to 40%, from 30 to 95 parts of talc, silica, mica, sillimanite, asbestos, clay, glass for electric insulation, synthetic mica, magnesium silicate, aluminium silicate, magnesium oxide, aluminium oxide or the like powder having a particle size of from 5 to 150 microns, and from 10 to 100 parts of boron oxide, lead oxide, antimony trioxide or the like, which is described in JP-B-39-8547, must be subjected to stepwise heat-treatment to be effected in the vicinity of 100.degree. C., then 300.degree. C. and then 600.degree. C. in order, after having been left to stand at room temperature for a while, when it is actually used for adhesion. Such stepwise heat treatment process is not simple but is extremely complicated. Additionally, the hardened product of the adhesive vitrifies at a high temperature and therefore the heat-resistance thereof is not sufficient.
The composition illustrated in the above-mentioned JP-A-48-93636, which is composed of an acidic silica sol and a crystalloid silicate liquid does not have a sufficient storage stability, and it could not form an adhesive having a high adhesion strength. On the other hand, the composition illustrated in the above-mentioned JP-B-56-38544, which is composed of an aqueous silica sol, an inorganic fiber and a refractory powder, could not form an adhesive having a sufficient electric insulating property.
Additionally, it has been found that the hardened products of the conventional adhesives adsorb water from the aerial atmosphere to thereby gradually lower the adhesion strength.
Where an adhesive containing a silica sol and a refractory powder is applied to adhesion of various materials, only solution of the above-mentioned problems on the adhesion strength, long-time storage stability and heat-resistance is still insufficient and the adhesive is desired to have further additional higher properties. For instance, when an adhesive is to be fully filled even to fine slits between substances to be bonded with the adhesive by the use of an automatic adhesive-injector, the adhesive to be used for the purpose is desired to have a high fluidity and a good workability. Specifically, the dispersion stability of the adhesive is indispensable so that the components of the adhesive do neither precipitate nor coagulate during handling it.
In addition, where an adhesive is hardened by drying, the hardened product is desired to be free from uneven shrinkage, cracking and foaming. That is, the adhesive is desired to have a good hardening property.
On the other hand, where the parts as bonded via an adhesive are exposed to temperature variation between a low temperature condition and a high temperature condition in practical use thereof, the adhesive used is desired to have an extremely high adhesion strength and additionally the hardened adhesive layer between the parts as bonded therewith is further desired to have a properly controlled thermal expansion coefficient. In particular, in an electric-light bulb or an electric heater, the joint part between the bulb glass or jacket glass to cover the heating element and the lamp base is exposed to such a temperature variation in an extremely short period of time, or that is, it is exposed to a so-called heat shock. Accordingly, the adhesive to be applied to such a joint part is indispensable to have a sufficient heat-shock resistance enough to be resistant to repeated heat-shock of the kind without breakage of the bonded joint part.
Additionally, where at least one part of the parts to be bonded with an adhesive is a metal material, the adhesive is desired to be free from corrosion of the metal material because of the hardened adhesive layer during practical use.